Video: Membranous organelles

After a busy day, sometimes you just can't be bothered cooking and it feels good to get some takeout. Once you place an order, the restaurant needs to prepare food, check it, package it, sort it, and send it out such that you get exactly what you ordered. It's a highly organized system.

But you know what's even more efficient? Our cells. The structures inside the cell work seamlessly together to prepare, check, package, sort, and send out material. And in this tutorial, we'll be learning about these efficient membranous organelles.

The cell is the structural and functional unit of all living organisms. Each cell consists of a nucleus and cytoplasm surrounded by a cell membrane, also known as the plasma membrane. Within the cytoplasm are organelles distributed in a gel-like fluid known as the cytosol.

The organelles suspended here perform specific functions, working like departments of a well-functioning factory. Some of the organelles are surrounded by membranes themselves, which keeps things organized within the cell. These membranes are similar to the plasma membrane as they are phospholipid bilayers.

Membranes regulate movement of molecules into and out of organelles, confine proteins and enzymes to the correct structure ensuring processes run smoothly, and isolate toxic substances. These membranous organelles, also known as membrane-bound or membrane-enclosed organelles, include the nucleus, endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, and peroxisomes.

Today we're going to be looking at these five organelles. We can split them into two groups: those that belong to the endomembrane system and those that do not.

Let's start with the ones outside the endomembrane system, which include the power plant of the cell, the mitochondrion.

This very popular bean-shaped organelle doesn't have just one membrane; it actually has two: an outer membrane and an inner membrane. Between these two membranes is the intermembranous space.

Within the inner membrane is the mitochondrial matrix, a gel-like substance with lots of enzymes. These enzymes are essential for biochemical pathways involved in food breakdown and metabolism, such as the citric acid cycle. The inner membrane has folds known as cristae, which increase the surface area, enhancing its ability to perform an important function: energy production.

The mitochondrion is called a powerhouse for a good reason. Its enzymes extract energy from food breakdown products in the form of adenosine triphosphate, or ATP. This is what the cells use as fuel. For example, muscle contraction requires energy from ATP produced by mitochondria. Such cells with high energy requirements have a larger number of mitochondria than those in the skin, which don't require as much.

The synthesis of ATP requires oxygen -- yes, from the very air that we breathe. It's known as oxidative phosphorylation and the enzymes required for it are located along the inner mitochondrial membrane.

Another organelle in the cell that uses oxygen is the peroxisome. Unlike the double-membraned mitochondria, these organelles have a single membrane. Within these organelles are oxidases, which use oxygen to oxidize molecules, such as very long chain fatty acids. This process generates a toxic byproduct, hydrogen peroxide, which is immediately broken down within the peroxisome itself by another enzyme known as catalase. Thus containing these enzymes within neat compartments is very smart; otherwise, that toxic hydrogen peroxide would be wreaking havoc in the cytosol.

Peroxisomes also use catalases to remove toxic substances such as ethanol from the bloodstream. That's why you can find a large number of peroxisomes in organs such as the liver and kidney where a lot of detoxification reactions take place.

Now let's move on to our next group of organelles: the endoplasmic reticulum, Golgi apparatus, and lysosomes, which together form the endomembrane system of the cell.

This system of organelles takes proteins and lipids and modifies, sorts, and transports them to their correct destinations. Let's see how they do that.

In the endoplasmic reticulum, a series of membranous tubules and sacs, known as cisternae, communicate with each other. The surface of the endoplasmic reticulum may or may not be studded with ribosomes.

The portion without ribosomes is the smooth endoplasmic reticulum, while the regions that have ribosomes form the rough endoplasmic reticulum. The membrane of the rough endoplasmic reticulum is continuous with the outer membrane of the nuclear envelope.

The smooth and rough endoplasmic reticulum are connected to each other but they differ in function. The smooth endoplasmic reticulum is where the cell synthesizes lipids and steroid hormones, such as phospholipids, which are the main component of membranes, and reproductive hormones. Thus smooth endoplasmic reticulum is abundant in the cells of the testes and the ovaries, which are the organs that synthesize reproductive hormones. This is also the site of detoxification of substances such as alcohol and barbiturates, making it an abundant organelle in liver cells.

Another important function is the storage and controlled release of calcium. For example, a highly specialized version of smooth endoplasmic reticulum, called the sarcoplasmic reticulum in skeletal muscles, can hold onto calcium for controlled release. This calcium plays an important role in muscle contraction.

On the other hand, the rough endoplasmic reticulum handles proteins. The actual synthesis of proteins happens in ribosomes, which could be free in the cytosol or bound to the surface of the endoplasmic reticulum and nuclear envelope.

The protein synthesized in bound ribosomes can get incorporated into the membrane of the endoplasmic reticulum or enter the lumen. In the lumen, they undergo modifications as they're transported through cisternae, like the addition of sugar residues, a process known as glycosylation.

The proteins are folded, and these modified proteins leave the rough endoplasmic reticulum in transport vesicles that are headed towards the Golgi apparatus. Also known as the Golgi complex, this organelle is another set of sacs and vesicles that looks like a stack of pita bread. However, unlike the endoplasmic reticulum, the sacs generally don't communicate with each other. The membrane around each stack separates the lumen of the Golgi apparatus from the cytosol.

This is the sorting and packing station of the cell. It has a receiving side, the cis face, and a shipping side, known as the trans face. Material from the endoplasmic reticulum reaches the Golgi apparatus in vesicles that fuse with the cis face.

There are multiple models explaining how cargo gets transported through the Golgi apparatus. The first is the vesicular transport model, where vesicles form and fuse with the cisternae at each step as they move from the cis to trans face. Here the cisternae are static.

The second is the cisternal maturation model, where the cisternae are more dynamic. They mature as they carry cargo from one end to the other.

As proteins and lipids travel through the Golgi apparatus, they are further trimmed and modified. This includes receiving tags similar to address labels, which determine their destination. Towards the trans face, the proteins and lipids are sorted and packaged into transport vesicles ready to enter the next phase of their journey.

Some vesicles have proteins that form components of the cell membrane. Some form secretory vesicles that contain material that is destined to leave the cell. They do so by a process known as exocytosis.

A third group of vesicles forms another cellular organelle known as the lysosome.

The lysosomes are the cleanup crew of the cell. They have hydrolytic enzymes limited by a membrane. These are most commonly acid hydrolases which digest and break down macromolecules.

For optimal enzyme activity, these spherical structures have an internal pH of around 5, which is acidic. Meanwhile, the cytosol has a pH of around 7.2. Any leaked enzymes are inactivated at this pH, protecting the cell.

When cells take up material from the extracellular environment by endocytosis or phagocytosis, these vesicles fuse with primary lysosomes to form secondary lysosomes, which digest the contents.

Damaged organelles form autophagosomes. Lysosomes recycle these organelles by fusing with the autophagosomes to form autophagolysosomes, another type of secondary lysosome. This process is known as autophagy.

The nutrients from the digested material are released through the membrane into the cytosol, while the undigested material is retained as a small residual body.

Thus the endoplasmic reticulum, Golgi apparatus, and lysosomes work together forming a well-functioning endomembrane system.

However, it's important to note that the nuclear envelope is also part of this system and we'll be learning more about the nucleus and its envelope in a separate tutorial.

Before we sign off, let's summarize what we've learned about the importance of membranes around organelles. They're essential for a variety of reasons, such as compartmentalization, keeping the proteins and enzymes necessary for a particular process confined to a specific organelle; protection, ensuring toxic substances are contained and don't leak out; maintenance of pH gradients, enabling optimal enzyme activity; regulation of molecular exchange between compartments by the selective permeability; and intracellular transport, by contributing to the formation and fusion of vesicles.

That concludes this tutorial on the membranous organelles of the cell.

Learn more about the cell with our study units and quizzes at Kenhub.